The real mystery is with the older bikes that we had before we had any kind of ways to track mileage. Sure, there were the old clicker things that tripped a lever with each turn of the wheel but every one we had in our family, eventually got lost or eventually taken off the bike for whatever reason.
I rode my old Sears bike for about fifteen years, until eventually the large chainring wore out to the point it would skip beneath the chain during hard pedaling. No idea of the mileage unfortunately.